1. Field of the Invention
The present invention relates to an ink jet printing method, wherein a recording medium is moved relative to a print head having a plurality of nozzles, the method comprising the steps of adding spit pattern data to print data to be printed by the print head, supplying the print data and the added spit pattern data to the print head, and ejecting ink droplets from the nozzles onto the recording medium in accordance with the print data and the spit pattern data in a single pass of the recording medium along the print head.
2. Background of the Invention
In ink jet printing, nozzle failures may be caused by nozzle clogging, contamination of a plate in which the nozzles are formed, events in which the nozzles are touched by the recording medium, and the like. Such nozzle failures are a serious threat to reliable ink jet printing and to print quality.
In a single pass print process, the print head and the recording medium are moved relative to one another in such a manner that each location on the recording medium is exposed to the nozzles of the print head only once. When the width of the print head is at least as large as the width of the recording medium, the recording medium may be moved past the print head in a uniform direction, or, conversely, the print head may be moved over the recording medium only once. When the print head does not cover the entire width of the recording medium, it is moved in a main scanning direction across the paper so as to print one or more lines, and the paper is then advanced in a sub-scanning direction, so that another swath of the image will be printed in the next pass of the print head. Such a single pass process is particularly vulnerable to nozzle failures because there are only limited possibilities to compensate nozzle failures by printing extra dots with other, still intact nozzles of the print head.
It is known that the risk of nozzle failures increases when a nozzle is inactive for a certain time, because the ink may dry-out in the nozzle. DE 10 2007 035 805 A1 proposes a multi-color ink jet printing method of the type specified in the opening paragraph, wherein the risk of nozzle failure is reduced by causing the nozzles to “spit” onto the recording medium from time to time even when the print data do not command a dot to be printed. In order to hide the extra dots from human perception as far as possible, the spit pattern is designed such that each extra dot will be superposed with a dot that is printed in another color, so that the extra dot is covered by a “regular” dot, or at least the extra dot does not significantly change the visual impression, because an ink dot, though in a different color, would have to be present at the dot location, anyway.
Another approach to improve reliability in ink jet printing involves an automatic nozzle failure detection, which permits taking measures for removing the nozzle failure before a larger number of defective images is printed. For example, nozzle failure may be detected by printing a test pattern and then inspecting the test pattern from time to time. However, this method implies a waste in paper and ink, especially when the test is repeated in short intervals. Moreover, this method requires a sheet disposal trajectory in the paper pass of the printer, so that the sheets carrying the test pattern may be disposed.
Another method of nozzle failure detection involves inspecting the image that has been printed in accordance with the print data. This has the advantage that a nozzle failure can be detected immediately, and the running print process may be stopped, if necessary. However, depending on the nature of the print data, it may be difficult to detect nozzle failures, and when a nozzle failure occurs at a nozzle that is not currently used for printing, the failure cannot be detected before the nozzle is used again.
U.S. Pat. No. 7,393,077 B2 discloses a method of nozzle failure detection wherein, in a first step, only specific ink dots that shall be used for nozzle failure detection are printed on the recording medium, these ink dots are then inspected for the purpose of nozzle failure detection, and then the inspected area of the image is moved past the print head in a second pass so as to print the rest of the image in accordance with the print data. Consequently, this method requires a multi-pass print process. It is further observed in this document that the dots for nozzle failure detection do not have to form part of the image to be printed in accordance with the print data, but should in any case be located in a low visibility area of the image, especially an area in which the spatial frequency of the image to be printed is within a certain range.